Stripping of chromium plate using a solution containing sulfamic acid and a water soluble chloride



United States Patent 3,262,775 STRIPPING OF CHROMIUM PLATE USING A SOLU- TION CONTAINING SULFAMIC ACID AND A WATER SOLUBLE CHLORIDE Charles F. Griflin, Jr., 10 McGrady St., Holyoke, Mass. No Drawing. Filed Feb. 26, 1964, Ser. No. 347,366 5 Claims. (Cl. 75-97) This invention relates to the removal of coatings or platings of chromium or chromium alloys from articles coated or plated therewith.

Frequently steel articles are given a temporary chromium coating intended for eventual removal, or the coating may be so defective as to dictate its removal. For these or other reasons, it is frequently desired to remove the chromium or chromium alloy coating or plating from a metal base, such as iron, steel, or other ferrous or nonferrous material, coated or plated therewith.

The removal or stripping process is normally efiectuated by electrochemical means and/ or the employment of a strong acid bath. With an electrochemical means, a solution containing a currrent-conducting salt or resistance-decreasing reagent is employed in order that the article to be stripped may constitute one electrode and an electrolytic action set up to cause the plating or coating to be mechanically loosened. But such technique, aside from being time-consuming, difficult to control, and expensive, has not been consistently satisfactory as respects the efliciency of the cathode to plate out the chromium and/ or as concerns the efficiency of the electrolytes employed. Liquid acids used for such stripping, on the other hand, have been notoriously slow, or have necessitated sacrifices in accuracy in order to gain speed. Worse, such agents are normally so highly acidic that they attack the base material as by etchingor pitting. Such acids have presented damage and waste problems such as commonly encountered in the use of muriatic acid with its attendant health hazards unless the stripping tank is well ventilated, especially under conditions of applied heat. Hydrochloric acid is known for use in accomplishing the stated purpose but is objectionable for the reason that it is exceptionally corrosive and attacks and dissolves the metal underlying the plate or coat to be removed unless a suitable corrosion inhibitor is added thereto to reduce the rate of attack.

I have discovered, and it is one of the teachings hereof, that improved techniques for stripping chromium plate are realized in the salient respect of stripping in the absence of electrical current and additionally in the absence of excessively high temperatures (in the range of 180 F. and above) by the use of a bath formulated from one of a plurality of certain combinations of materials.

It is a primary object of the invention to provide an improved method, and improved formulations for use with the method, in connection with the complete and rapid stripping of chromium and chromium alloys from steel and other metal bases coated or plated therewith, wherein the need for the use of electrolytic means and high temperatures, as required in the case of alkaline chromium strippers, is obviated to the end that the stripping procedure is advantageously simplified.

Another object hereof is to provide a comparatively inexpensive one-step procedure for effecting rapid removal of a coating or plating of chromium or chromium alloy from a ferrous or non-ferrous 'base without undue attack upon the base and/or without producing excessive detrimental etching effects so as to allow a stripped reactive surface which will readily accept a replating with chromium.

The invention is predicated on the discovery, after exhaustive experimentation with various formulations, that ice by including sulfamic acid and a water soluble chloride in an aqueous solution, chromium may be removed from a ferrous or non-ferrous metal surface with greatly reduced attack upon the underlying metal as compared to the result obtained when other acid solutions such as hydrochloric acid are used.

Sulfamic acid will not, itself, effectively attack chromium with any degree of consistency and l have determined that a compatible salt may be cooperant therewith so as advantageously to change the character of the chromium coating or plating in manner such that the sulfamic acid is rendered effective. A formulation of sulfamic acid and compatible salt will not etch the metal, such as nickel, for example, so as to dull the surface thereof; quite to the contrary, it will allow a high or extremely bright finish corresponding to the original condition of the metal treated therewith.

Sulfamic acid is an organic sulphur acid of the general formula:

I HnN- S-OH Although any suitable water soluble chloride and sulfamic acid may be selected, it is obvious that that the two must be compatible. I have found that sodium chloride or potassium chloride are suitable chlorides for combining with sulfamic acid in an aqueous solution.

The interaction of the compounds liberates the chloride ion from the chloride and the sulfamic ion from the acid which cooperantly function to insure a full reduction of the chromium coating or plating.

Other chlorine compounds which are soluble in sulfamic acid and do not contain an element which, will inhibit or impair the reduction action, can be substituted for the sodium chloride or potassium chloride above itemized.

To assure optimum results, the ratio of sulfamic acid to chloride is 2: 1, but may vary from 4:1 to 2:2 and still provide proportionately effective solutions.

Preferentially, but not obligatorily, catalysts, such as copper sulfate and/or manganese sulfate, may be used.

Copper sulfate functions as a catalyst to accelerate chemical action, and additionally as an indicator insofar as optimum temperature conditions are concerned in that at temperatures below F., it contributes a reddish color to any solution, which reddish color disappears at temperatures thereabove.

The copper is displaced from the solution by chemical decomposition below 130 F. As the temperature rises,

the hydrogen in activity is sufficient enough to prevent the The same can be accomplished by the use of copper hooks or baskets.

As the generated hydrogen gas has a tendency to stick to the base surfaces, manganese sulfate in the solution acts as a depolarizer by eliminating the hydrogen from adhering to the surface.

Further, in the event of any tendency of the solution to become passive, the manganese sulfate advantageously serves to prevent dormancy.

Normally, with the solutions hereof, stripping action is initiated within 60 seconds. Nonetheless, it is not uncommon that the chromium may be so passive that the stripping reaction does not commence within such time period. In such event, it is necessary only to lift the Work from the solution, expose it to the air for a few seconds, and reimmerse therewithin.

3 The incorporation of a wetting agent, preferably an anionic, will help to minimize the liberation of gases to the atmosphere.

'The following tabulation shows the preferred range of bath composition and operating conditions and typical examples:

Preferred Example Example Example Range 1 II- III Bath Ingredients:

Sullamic Acid, oz. per

gal 6 to 12 9 6 12 Water Soluble Chloride, oz. per gal 2 to 4 3 3 4 Wetting Agent, oz. per

gal 0.1 to 0.5 0.32 0. 3 0. 4 Copper Sulfate, oz. per

gal 0. 0005 0.01 0.01 0. 01 5 Manganese Sulfate, oz.

per gal 0. 0005 3% O. 01 0. 01 0. 01 Water Balance Balance Balance Balance Operating Conditions:

Bath Temperature,

F to 150 145 145 The mixtures of the examples will dissolve chromium plate without current while operating within a wide temperature range, although the optimum temperature range from 140 F. to F. has been found to allow most rapid and complete stripping. While the temperature may be permitted to rise above the stated temperature range, its effects are noted in a greater tendency towards decomposition. Temperatures within the preferred range favor eflicient stripping, with an avoidance of decomposition.

In contrast to hydrochloric and other liquid acids, the compositions hereof are dry mixtures during shipment and storage, making them less dangerous and easier to handle and use, being convertible to solutions when ready for use by adding water to the mixture and stirring. The optimum concentration is 1 to 2 pounds per gallon however, it will work at lower concentration.

The high level of fumes as found with liquid acids is greatly reduced. I V

V In using the improved strippingcomposition, it may be brought into contact with the surface to be stripped in any suitable manner, as by immersing the article bearing the plate in the composition or otherwise inundating the plated surface. Stripping usually is accomplished in a few minutes depending upon the nature and composition of the plate. It is preferable to use the composition at elevated temperature, such as from about 130 F. to

F., although other temperatures may be used. The lowest temperatures at which reasonably rapid stripping is had are used preferably.

It will be apparent to those skilled in the art that some variations in the described procedure may be employed without departing from the spirit and scope of my invention as indicated by the appended claims.

What I claim is:

1. In a metal treating process, the steps of removing the chromium from a metal coated therewith which com prises, immersing the coated metal to treatment in an aqueous solution comprising essentially dissolved sulfamic acid and a water soluble chloride in water, at a temperature within the range of 130 F. to 160 F.

2. A method of the character described for removing chromium from a metallic base which comprises, immersing the coated metal in an aqueous solution consisting of sulfamic acid and a water soluble chloride, the chloride being in a minimum amount of 1 oz. per gallon of water to act upon the coating and make it subject to effective action by the sulfamic acid, the sulfamic acid being in a minimum amount of 3 oz. per gallon of water.

3. The process of stripping a chromium plating or coating from an alloy or metal article which comprises, immersing the article in an aqueous solution containing sulfamic acid and a water soluble chloride in effective amounts sufficient to effect the stripping action.

4. The process of stripping a chromium plating or coating from an alloy or metal article which comprises, immersing the article in an aqueous solution of sulfamic acid and a water soluble chloride in amounts and at a temperature sufficient for effecting a stripping action.

sulfamic acid and a water soluble chloride wherein the chloride and sulfamic acid are in amounts sufficient for effecting the stripping action.

References Cited by the Examiner UNITED STATES PATENTS 3,171,767 3/1965 Bellinger et al 252 79.4

FOREIGN PATENTS 896,159 5/1962 Great Britain.

DAVID L. RECK, Primary Examiner.

O N. F. MARKVA, Examiner. 

1. IN A METAL TREATING PROCESS, THE STEPS OF REMOVING THE CHROMIUM FROM A METAL COATED THEREWITH WHICH COMPRISES, IMMERSING THE COATED METAL TO TREATMENT IN AN AQUEOUS SOLUTION COMPRISING ESSENTIALLY DISSOLVED SULFAMIC ACID AND A WATER SOLUBLE CHLORIDE IN WATER, AT A TEMPERATURE WITHIN THE RANGE OF 130*F. TO 160*F. 